In a conventionally known device and method for controlling an electric motor that includes a plurality of energization systems each including an inverter and coils corresponding to different phases, two inverters may be used to supply a current to a single electric motor which generates a steering assist force in a vehicle electric power steering system, as illustrated in Patent Document 1, for example.
In such a control device and method, when one of the inverters fails, the output of the failing inverter to the electric motor is force-stopped while the other inverter, which is normally operating, is caused to provide an output whose amount is equal to that normally provided by the two inverters so as to prevent the total output from all the inverters from dropping and thus to prevent the required steering force from abruptly increasing. A conceivable measure to make this possible is to approximately double the tolerance of each inverter, which, however, leads to an increase in production costs.
Thus, in the device and method for controlling an electric motor disclosed in Patent Document 1, when one of the inverters fails and the output of the failing inverter is force-stopped, the output of the other inverter, which is normally operating, is controlled as follows. Specifically, at first, the normally operating inverter is caused to supply the electric motor with a current twice as large as the current the inverter supplies when both the inverters operate normally, and, after a predetermined time passes, the current that the normally operating inverter supplies to the electric motor is gradually reduced from the viewpoint of overheat protection.